Sustainable mortars using overburden-derived M-sand and coal bottom ash: Mechanical properties, durability and life cycle assessment

The scarcity of river sand (RS) and the rapid accumulation of mining overburden and industrial by-products such as bottom ash (BA) pose significant environmental challenges. Excessive RS extraction degrades aquatic ecosystems, while waste disposal contributes to land degradation, highlighting the need for sustainable alternatives. This study explores replacing RS with manufactured sand (M-sand) produced from mine overburden sourced from Bokaro and Singrauli (BS and SS), along with partial substitution of M-sand by BA in mortar mixes. Mortars were prepared at a sand-to-cement ratio of 1:2.75 with 0–30 % BA substitution at water-to-cement ratios (w/c) of 0.50 and 0.60. Workability was evaluated using flow tests, while mechanical and durability performance was assessed by evaluating compressive strength, ultrasonic pulse velocity (UPV), sulphate attack resistance, rapid chloride penetration (RCPT), and accelerated carbonation. Microstructural analyses (FESEM and XRD) were conducted on the mortar mixes to investigate the hydration behavior, pore refinement, and BA’s role in enhancing interparticle bonding. Results indicate that optimum BA replacement is 20 % for BS-based mortars, while both SS and RS-based mortars exhibited compressive strength gains up to 30 % BA replacement, indicating this as an effective replacement level. BA addition significantly improved durability by refining pore structure and reducing permeability. Life Cycle Assessment (LCA) revealed that M-sand based mortars, with 20–30 % BA substitution, minimize riverbed mining and promote productive utilization of overburden and industrial waste. Although M-sand mortars were marginally costlier (∼ 3–4 %), they represent a technically viable, durable, and environmentally sustainable alternative to RS in mortar applications. • M-sand from coal mine overburden is a sustainable alternative to river sand. • Coal bottom ash (BA) substitution (20–30 %) improves strength and durability. • Optimum mortar mix with compressive strength (32–43 MPa) and enhanced durability. • M-sand mortars are 3–4 % costlier but offer major environmental benefits.